1. Field of the Invention
This invention relates to a process for treating sugar-factory molasses comprising dilution of molasses with cold water, precipitation by addition of quicklime of the saccharose of the thus diluted molasses, and then filtration of the so-treated molasses. This process is well-known as the cold precipitation step of the Steffin process.
2. Description of the Prior Art
It is generally known that in order to cold precipitate by means of quicklime the saccharose of sugar-factory molasses, such molasses has to first be diluted by addition of cold water to a polarization of 6.
Polarization is the value which is commonly used in a sugar-factory in order to characterize the sugar content of a sugar liquor. As a matter of fact, the polarization represents the saccharose concentration, in gr. % cm.sup.3, calculated from the measurement made with a polarimeter by supporting that the liquor only contains saccharose as the optically active substance. It is quite certain that technical sugar liquors contain a number of optically active impurities, but, however, their effect is only noticeable at very low saccharose concentrations. It is even possible in this case to reach negative polarizations, which means that substantially no saccharose still remains in the liquor. Within the scope of the present disclosure, the term polarization will be always used in order to characterize the sugar content. It has been remarked that when molasses having a polarization higher than 6 is reacted according to the cold precipitation step of the Steffen process, the results obtained become unsatisfactory.
More particularly, an increase in the sugar content of the filtrate, the so-called cold filtrate, is noted, this filtrate being obtained after filtration of the insoluble saccharose-lime combination formed due to the action of the quicklime, as well as a substituted reduction of the filterability of this combination.
Sugar-factory molasses presents a polarization which generally varies around 68. In order to decrease the molasses polarization to a value allowing use of the cold precipitation treatment, a lower molasses polarization at the beginning of the treatment must be obtained, and a greater amount of cold water has to be added. It is advisable, however, to reduce as much as possible the amount of cold water which is necessary. This is due to the fact that this allows for a reduction in the frigorific group power and also allows for a decrease in the cold filtrate volume. That is to say, for a residual saccharose concentration which is admitted in the cold filtrate, the product of the residual saccharose concentration into the volume of cold filtrate decreases ; in other words, the yield of the precipitation process increases.
Furthermore, all of the molasses impurities concentrate in a smaller cold filtrate volume, which makes concentration by evaporation more economical and allows for an improved price to be obtained as animal food.
It has already been proposed to decrease the cold water amount which is necessary for dilution of sugar-factory molasses by recycling a part of the cold filtrate. However the recycled cold filtrate volume/ molasses volume ratio cannot exceed in the known processes a very low limit beyond which the sugar losses increase and the filterability of the insoluble saccharose-lime combination decreases to practically unacceptable values. Also, in various patents of the applicants, such as Belgian Pat. Nos. 775,564 and 791,033 on the one hand, and 735,802 and 752,422 on the other hand, improvements have been proposed to the Steffen process which have as their effect allowing diluted molasses having a polarization higher than 6 to be treated during the precipitation step. According to the first-mentioned group of patents, the treated molasses is decanted before filtration and a part of the decantation product is recirculated to the diluted molasses, before addition of the quicklime thereto. According to the second-mentioned group of patents, a part of the treated but not yet filtered molasses is recirculated to the diluted molasses before addition of the quicklime thereto. These known processes have already produced for a reduction to some extent in the fresh water amount which is used to dilute the molasses while maintaining an optimum ratio between recycled cold filtrate volume/molasses volume before dilution.